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Can passenger movement in an airliner make it stall?
This question shows a couple of air hostesses dancing during mid-flight and the co-pilot came out of the cabin to record it.
This made me wonder:  Can flight crew or passengers moving around in flight cause the airplane to stall?

I used to teach sport parachuting, and we occasionally pranked the pilots by shifting the weight around.
A Cessna 206 set up for parachuting will have all but the pilot's seat removed, the jumpers sit on the floor. Everyone moves forward for takeoff and then sits back for the ride up. The instructor (me) sits on the floor where the co-pilot would normally be, facing aft, everyone else faces forward. I would wait until the plane was trimmed for climb (and well above 1000 feet) and then motion the passengers to come forward a bit (the pilot cannot see this happen). He adjusts trim as usual, thinking nothing of it. Passengers come forward some more. Trim adjusted again. Eventually we have 5 people wearing 20kg packs all jammed up into the first 2 "rows" and the trim almost fully nose up.
One signal, and 1/3 of the airplane's gross weight quickly moves almost 2 meters aft (in a 4 meter cabin), followed by a sharp pitch up in both the nose and the pilot's blood pressure. The stall alarm usually goes off too.
Mid-size tailgate aircraft used for jumping are regularly stalled by too many people on the tailgate for too long. The rule there is stay forward until it's your turn, then go aft and get out (if the buzzer goes, you get out NOW).
So it's possible to stall a small aircraft by moving the passengers around.
Scaling this up to airline sizes, we find that the passengers are a rather smaller percentage of the airplane's gross weight, and the presence of things like seats, bulkheads and galleys at the rear prevent 300 people from gathering rapidly at the back. The rather large hydraulically powered elevator also has substantial authority over the aircraft's attitude.
So no, a small group of passengers won't be stalling an airliner out of the sky no matter where they stand. Noticed by the pilots: possibly.

A stall warning at 1000ft with a fully loaded plane does not sound entertaining at all all right :O

@yankeekilo - probably depends on if you are planning to jump out of it anyway!

Notice I said "well above 1000 feet". And any pilot who can't recover from a soft stall in a jump-modified airplane shouldn't be flying. ( biggest engine and prop available, leading edge cuffs, flap-gap fillers, winglets - every high-lift mod on the market)

@Paul Just because they should doesn't make this a good 'prank'. Do you prank your friends by pulling up on the handbrake as they drive along the road too?

@Cloud I know some friends doing this in winter on snow. Good prank is a mix of good theme, perfect timing and good target. You can do it to someone you really knows good enough.

Theoretically? Yes.
Practically? No.
Passengers moving around in an aircraft will have some effect on the balance (center of gravity) of that aircraft, but the proportion of weight for a few flight attendants or passengers moving around on a modern airliner is likely so small as to be negligible. It's not enough of a change to exceed the control authority of the aircraft and cause it to pitch up enough to stall (unless as mah notes you have a substantial number of extremely heavy folks on board and they all stand right up against the aft bulkhead - and even then it's not likely).

I can't find the link to it, but I recall either reading about or hearing about a mostly-empty flight (I want to say on some kind of smaller multi-engine piston aircraft)in in which the half-dozen or so passengers (having had a few "adult beverages") decided to play a friendly prank on the pilots by slowly moving to the furthest aft seats, waiting there for a while, and then slowly moving back to the furthest forward seats.
As I recall the story the aircraft in question had no autopilot, so the pilots had to manually re-trim the aircraft after each round of seat changes. The end result ultimately being that the pilots announced that everyone had to pick a seat and stay in it, or the beer cooler would be locked for the remainder of the flight.
(As you can probably imagine there was no further need to re-trim due to unexpected CG changes following that announcement.)

In large aircraft, practically no. In a commuter, see Filair L410 at Bandundu on Aug 25th 2010, impacted building.

I remember a post on CompuServe of a veteran pilot who ferried live pigs in Vietnam. One of the pigs broke its cage and started running up and down the cargo hold. This excited other pigs, an soon he had a number of them running around, all together. In the end, he had to lower the rear cargo ramp and let them go in mid flight.

@PeterKämpf . . . I think the important question is "Does this count as pigs flying?"

:-) yes, flying along a ballistic curve ...

I know almost nothing about aviation. Can someone please explain to me what "trim" and "re-trim" mean?

@GeorgeMillo That would make an excellent new question :)

I very much doubt a group of random people would know that moving has any affect on the aircraft, unless they were all pilots...

There was a crash in the Congo that is presumed to be caused when a loose crocodile in the cabin sparked panic, and catastrophically shifted the balance of the aircraft. Not something you hear every day.

What's up with this referring to accident news from mainstream media that rarely verify their facts or update the stories? The crash you mention, Filair L410 at Bandundu on Aug 25th 2010, impacted building, is indeed suspected to have been caused by too many passengers moving, but the survivor later changed their testimony regarding the reason.

@JanHudec Thanks for clearing that up.

@JanHudec On the other hand, the account in the article you link also doesn't make sense. If you look at Bandundu airport on Google maps, there is clearly no "reserve strip" at the side of the runway, just a bunch of scrubby land about 150m wide on each side. But even if a temporary grass strip had been prepared, how would the passengers be able to tell, more than 2km from the runway that the crew were off-line by only 50-100m?

@DavidRicherby: Indeed, that story does not make sense. And after more than three years, there does not seem to be any outcome from official investigation either.

In short possibly...
But only if the aircraft is already heavily laden and there is a large mass shifting all at the same time. In the case of parachutists on a bare floor with no restraints, yes of course.
This 747 took off from Bagram airfield in Afghanistan with five armoured vehicles and they are thought to have broken free and shifted aft during take off:

Business Insider article

An aircraft stalls when it exceeds its critical angle of attack.
Unless the dancing flight attendants all gather aft, and are much heavier than any normal human could be (and even then), I think it's not possible for them (dancing, or being wall flowers) to modify the aircraft's angle of attack.
In short: no.
Further information on aircraft stalls: http://en.wikipedia.org/wiki/Stall_(flight)

I think it's a very interesting question (related one here). Let's look at an extreme example:
For an airliner, if all pax and cabin crew move rapidly in a coordinated manner to the extreme aft or front, could they take the airplane out of CG limits? Make it uncontrollable? (So, not referring to small/medium aircraft, or a few pax moving randomly or to the lavatory, which obviously doesn't endanger the aircraft.)
I'd love to see a more thorough answer, idealiter with reference to a POH or Weight & Balance manual, but here's a shot for, say, an A320:
Total mass is about 80 tons, the pax are about 15 tons (200 x 80kg). Length of the aircraft is some 38m. If everyone moves from their seat to the aft, they move (on average) around 16m (32m cabin length, say, and half of that because some move more, some less).
Thus, the CG moves by $15 \text{ t} \cdot 16 \text{ m} / 80 \text{ t} =  3 \text{ m}.$ The mean aerodynamic cord of an A320 is about 4.2 m (see discussion here, #7 or EASA here), so this represents a 70% MAC CG shift, or (if I understand it correctly) around 240 IU move. (Note that when all pax run to front and then to back, we'd have a 140% MAC shift).
As far as I can tell, the A320 has CG limitation of (at most) 15% to 45% MAC (see e.g. here, p. 81), thus accommodating at most a 30% MAC shift. So, it seems to me that the answer is yes, coordinated pax movement could take an airliner out of its certified CG load. Whether it remains controllable, I don't know.

Just because the plane is out of CG, it doesn't mean the plane will stall. With aft CG, the danger is thatIF the plane stalls, there is not enough forward elevator authority to break the stall.

@rbp What happens if CG is moved past CL (centre of lift)? Now tail horizontal stabiliser lift is upward, aren't we now statically unstable?

A stall is defined as exceeding the critical angle of attack of the wing.

Airflow defines how the tail plane flies, not stability

@rbp I think you're being slightly pedantic but you are correct. Being unstable doesn't mean you have stalled but it makes it a LOT more likely.

Note that my answer fits the related question I linked to (Can pax bring plane out of CG envelope?), which was marked as a duplicate of this question. As I said, I don't know whether the out-of-CG condition can lead to a stall or instability or uncontrollability. However, as pax (according to my rough calculation) can move CG out of its envelope by a considerable margin (2x), it's not inconceivable. Would be interesting to have an answer to that question beyond purely semantic distinctions.


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Is it possible for more than one airplane intercept the ILS at the same time?
I was watching a timelapse video yesterday of the approaches on a busy airport (I don't remember which one), and I saw that about 3 or 4 planes were lined up and descending at the same time (they were far from each other) and that got me thinking:

Is it possible (from a technical perspective) for more than one airplane to intercept the ILS signals (glideslope and localizer) at the same time?

Basically the ILS is a radio signal emitted by ground facilities that does not care about whose receiving it and does not react to receiver's action (like TV signals, except it is directional). Thus the only limitation on the number of users/receivers is possible interference.

The question is not as simple as it seems, reflection will occur on preceding aircraft, and Doppler shift too. Reflection may change the relative strength of the dual frequency beams, but Doppler may allow to exclude reflected signal them from reception. I really don't know the impact, but not simple. On the other hand, it is done.

@SteliosAdamantidis I'm more interested on the technical perspective. :)

See a mention to possible interference by in-flight aircraft, page 25-18 of Electronics Engineer's Reference Book

Related question:  "Can more than one listener hear the same FM radio station at the same time?"   The ILS issue is exactly the same:  It puts out a radio signal.   There is no limit to how many people can receive it.

@abelenky You're mostly right, but it is possible that an aircraft in front could be blocking and/or reflecting the signal. Just like normal radio stations, the waves propagate in a (more or less) straight line away from the transmitter and they don't travel through an aluminum aircraft very well (and they do reflect off of aluminum aircraft.) This is why we have to keep planes on the ground from taxiing in front of the localizer or glideslope transmitters while another aircraft is using the ILS for an approach.

@reirab:  I think the analogy is very strong.  Just as with the FM radio stations, some listeners may be blocked, or may hear reflections or interference. At extreme distances, the signal may be weak.  For an FM listener, its annoying.  For an airliner, its a safety hazard.

@abelenky Oh, yes, I agree in that regard. Certainly, the signals propagate and are subject to interference in (mostly) the same ways. There are a few forms a interference that are different with AM vs. FM (amplitude distortion doesn't matter with FM, but it does with AM and ILS is AM,) but it's mostly the same. The biggest difference between this scenario and the scenario of a normal radio station is that the 'listeners' in the case of an ILS are all lined up with each one between the transmitter and the 'listener' behind them... and that all of the 'listeners' are giant pieces of aluminum.

@abelenky: Actually there is also a need to get the DME information, the DME is an interactive device which cancels the argument of "receive only is unlimited" (which it is not by the way).

Yes, since the ILS is just a set of radio signals emitted and received by aircraft, there can be more than one aircraft established on localizer and glidepath. The ILS does not lock or tune onto one single aircraft, it continuously broadcasts the localizer and glidepath signal.
Since the localizer and glidepath antennas are located at the end of runway for the localizer and at the side of the runway for the glidepath, you will only need to worry about interference by aircraft on the ground, where they are close to the antennas and can deflect the signal due to proximity. This is why there are protection areas and aircraft need to hold at certain holding points, e.g. CAT II holding points when CAT II approaches are in use.
Aircraft in the air can also interfere and deflect the signal, however the emitted signals spread in a cone, so other aircraft still can receive the correct signals.
To quote KeithS from one of the comments:

[...] basically, the localizer is projected from the far end of the runway, and the glideslope is projected from the side of the runway at or near the touchdown target. Thus, aircraft in line to touch down on the ideal target will have good line-of-sight to both transmitter pairs (unless there's an aircraft in the ILS Critical Area)

(Image Source: www.aopa.org)

(Image Source: nustyR AirTeamImages (found via LondonReconnections))

(Image Source: When is an aircraft cleared to land?)

Is that top image real? Those airplanes look way too close to each other, unless there's some weird zooming illusions going on here. It doesn't look like there would be any way for the one in front to clear the runway before the next one is at DH. It doesn't even look like the one in front would be on the runway before the next one gets to DH.

I think the image is from LHR, where they use pretty closely packed spacing, especially in a headwind (http://www.nats.aero/newsbrief/time-based-separation-heathrow-world-first/)
Here is a video, that shows much the same spacing as the photo - https://www.youtube.com/watch?feature=player_embedded&v=fgHjVvqLXV8
And speculation, I don't know for sure -- but LHR has two close, parallel runways, so perhaps the following plane goes to the other runway in case of difficulties?

The 3 nm sep has nothing to do with ILS and planes can be right next to each other and receive ILS fine.  In IFR flight and in approaches (and in most other operations), planes are separated by distance and/or altitude to avoid collisions and wake turbulence problems.  Wake turbulence is particularly lethal on approach.

This is a good answer, but it doesn't explain why aircraft lined up right behind each other on the glideslope can still receive the signal with minimal interference. Some information about the placement of the transmitters on the runway complex might be useful; basically, the localizer is projected from the far end of the runway, and the glideslope is projected from the side of the runway at or near the touchdown target. Thus, aircraft in line to touch down on the ideal target will have good line-of-sight to both transmitter pairs (unless there's an aircraft in the ILS Critical Area).

The other thread mentioned optical compression...

@reirab, the photographer is using a strong telephoto lens, and is probably several miles the other side of the runway from the airplanes.  Judging from the atmospheric blurring of the two rear airplanes, I'd estimate at least two miles of separation between each.

@Eoin An aircraft on final will not change to another runway unless it encounters an emergency requiring it to land immediately and the runway in front of it is unavailable. In the case than an aircraft on final did declare an emergency that required them to land immediately, the aircraft in front of them would probably go around. If the runway is not clear when an aircraft on final reaches DH, it will go around.

@BrockAdams Correct, the 3 NM are ATC requirements, not ILS requirements.

@KeithS I edited my answer, provided more info and quoted something you mentioned in the comments. Anything else you would suggest to be added to make this answer better?

@reirab, most likely it's real. A long telephoto lens will compress distance quite a bit, and it's likely this was shot with a 600mm or longer lens.

If you look at the following picture, you'll see that a typical Localizer can giving reliable signals up-to 18NM from its position.

On the other hand, a typical Glideslope can give reliable signals up-to 10NM.

That said, it is absolutely possible to have multiple-aircraft conducting the approach right behind each other as long as all aircraft receive positive guidance.
Needless to say, traffic separation has to be maintained according to the regulations.

This, I think, is the better answer to the OP as it illustrates that the ILS signal is not broadcast from the spot on the runway where the planes are aiming (which is a common misconception). The glideslope component is projected from the side while the localizer is broadcast from the opposite end of the runway, and on approach you typically have good line of sight to both points even with a plane in front of you.

@KeithS that's right. To be more precise, the Glideslope is located in a building 750-1250ft from the approach end of the runway and 250-650ft from the centerline, and the localizer is located 1000ft behind the departure end of the runway.

An ILS is a beam of radio energy pointed down the approach path, there's no limit to how many aircraft can be using it from a technology point of view. In practice you will only have a few descending on the ILS at the same time due to separation requirements.
As you could see the 4 airplanes on approach it means that conditions were visual, so the airplanes weren't necessarily using ILS for approach. It's probable they were all approaching visually.

As mentioned by casey here the SOP often requires that if ILS is available, it must be used even for visual approach.

"there's no limit to how many aircraft...": For the sake of accuracy, there are limits that can be ignored up to a certain level, the DDM (difference of depth of modulation) must remain within ICAO limit. Outside airfield protected/critical ILS areas, wind generators, preceding and departing aircraft all create a multipath reception which affect DDM.

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What is the difference between Turn Coordinator and Artificial Horizon?
I was reading this question about Turn Coordinator and Artificial Horizon, and now I want to understand the differences between those two components.

What is the main difference between Turn Coordinator and Artificial Horizon?

Turn Coordinator:

Artificial Horizon:

Attitude Indicator
It shows your attitude (nose up / down, bank left / right) relative to a horizontal surface. Basically, it tells you the orientation of the plane if someone were to take a photo of the plane from the outside at this very moment.
It does not tell you which way you are going (e.g. you can be nose up but descending).
Turn coordinator
This is actually two instruments in one: the white plane and the little ball.
The white plane tells you which way you are yawing (rotation about the normal (vertical) axis)). If you are yawing to the right (e.g. N -> NE -> E), the right wing of the little plane is lower. If you are yawing to the left (e.g. N -> NW -> W), the left wing is lower.
If you are yawing at the standard rate (3 degrees per second), the wing of the little plane will align with the white mark.
Combining the two
Suppose that you manage to yaw the plane to the right while keeping it horizontal, like a spinning top:

You will get:

What is the function of the ball on the turn coordinator? Why does the iconography of the turn coordinator imply roll if it's measuring yaw?

@RussellBorogove The initial display used on instruments of this type was a simple arrow pointing to marks indicating "L" and "R" at the top of the instrument. This is called a "turn and slip indicator". That display style is much more intuitive IMO, but the original mechanism behind it didn't respond to banking maneuvers, which were convenient to manage banking rate. That was solved by canting the internal gyroscope 30 degrees, which made the gyro sensitive to rolling. Because the instruments respond differently, it's important to differentiate them, so the "turn coordinator" got a new look.

The function of the ball is to show if you are in a coordinated turn. Because just about every control surface interferes with the others when in use, the plane can enter "slips" or "skids" in which it is not turning as efficiently as it should. Rudder use can correct this, and the ball makes this easy to notice and deal with. The lines show where the ball should be in straight and level flight, but it serves a second, possibly unofficial purpose, for twin-engine planes in the event of engine failure. I'm not an MEL-pilot, so I don't want to go any further to avoid misinformation.

The primary difference is labelled right on your image of a TC: "NO PITCH INFORMATION". The artificial horizon is basically a pitch/bank position indicator, while the turn coordinator is a yaw/bank rate indicator. The horizon thus shows you how your aircraft is oriented right this instant, while the TC shows you how fast your orientation and/or heading is changing over time.
First, a correction from some other answers. A Turn Coordinator is not the same instrument as a Turn/Slip Indicator. They show similar information and are used interchangeably in normal flight, but they indicate slightly different things.
This is a Turn/Slip Indicator:

Notice the much simpler display. When the bar on the needle lines up with either of the home-plate symbols, you are yawing at a two-minute turn rate. No confusion with bank angle or any other indication of attitude.
The gyroscope in this device is basically rotating in a plane parallel to the aircraft's wings, and precession of this gyroscope induced by the aircraft turning is what drives the needle. However, a pure banking maneuver to start a "coordinated" turn doesn't show on the gauge; the aircraft has to start rotating relative to its up/down axis. This can make for a rough ride when attempting to bank to begin a turn, especially if that turn is being initiated by an autopilot; there's no gauge on board the aircraft to measure roll rate.
To solve that problem, the gyroscope in the instrument was canted 30 degrees in the plane from nose to tail; this makes the gyro sensitive to wing roll rate as well as fuselage yaw rate. This new version, the "turn coordinator" allows autopilots (and human pilots) to execute smoother turns in IMC.
However, there's a downside. Back in the 1920s, flying into a cloud, especially a large one, was often fatal; there was no artificial horizon yet, so once the pilot could no longer see the real horizon, if the plane's attitude were upset he'd be unable to recover. Then a pilot, Howard Stark, worked out a simple method using only the TSI and altimeter to recover from an upset; use the rudder to center the yaw needle, then ailerons to center the ball (with neutral yaw, the ball indicates the direction of gravity), and finally level out of any climb or dive until the altimeter is steady. This became known as the Stark 1-2-3 method and it saved a lot of lives.
The new "turn coordinator", because it responds to bank as well as yaw, will show you "yawing" again once you proceed to "step 2" and try to level the wings. Therefore, you can't use the Stark method with a Turn Coordinator. To indicate this and thus prevent pilots trying to use it as such, the display of the Turn Coordinator was changed to the symbolic aircraft:

The display makes some sense as the gauge will respond to roll rate, so it can be thought of as a combination roll/yaw rate indicator.
The modern method to keep an airplane in stable flight in IMC instead involves the artificial horizon:

Using a gyroscope mounted in gimbals, calibrated and subtly corrected in flight to rotate parallel to the ground (on an axis parallel to gravity while in level flight), it becomes possible to determine your exact angle of pitch and bank using a simplified representation of the real horizon. Blue is sky, brown is ground. Pull up, you see more sky, less ground. Nose down, just the opposite. Bank left, horizon rotates right, and vice versa, all like the real horizon out the window would be doing if you could see it. The instrument, as you can see, has markings indicating definite angles of pitch and bank.
Now, to recover from a minor upset in IMC, you simply pitch and bank to return the horizon to level, then while the horizon is stable, the Turn Coordinator will show you if you're yawing and how to correct it. This reduces the need for a pure yaw indicator in most normal situations. Extreme maneuvers, however, can render the AH unusable, so specialized "spin recovery training" is required to teach pilots how to recognize and quickly recover from extreme instabilities such as spins (uncoordinated stalls) and spiral dives.

Turn Coordinator
It is actually a turn and slip indicator. It tells you if you are:

turning or not
slipping or not
skidding or not

Artificial Horizon
This, the attitude indicator, tells you:

what is the pitch angle of the airplane when ascending or descending
what is the bank angle of the airplane when turning

The pull to cage know will cage the instrument, that means it will take its current pitch and bank as the new zero-reference position.
In glass cockpits or glass cockpit apps, many flight instruments are combined. It looks like this:

GlassCockpit 1000
The horizontal bar below the white triangle on the top arc is the slip/skid indicator.

To read more about aircraft turning angles/axes, please click here.
To read more about flight instruments, please click here.

You may hear the terms "turn coordinator" and "turn and slip indicator" used as synonyms, but they are slightly different instruments. A T/S indicator positions the gyro parallel to the wings, and only responds to turn (yaw) rate. A TC cants the gyroscope axis 30 degrees, which allows the indicator to respond more quickly to banking, which allows autopilot systems to control the bank rate using this instrument. It's important to know which one you're using, because, for instance, you can use the Stark 1-2-3 method with a T/S indicator but not a TC (the TC will flail around during "step 2").

I've flown a lot of different glass cockpit systems but I've never seen anything that looks like that. That can't be a real aircraft can it? dB's? Humidity? Light? It almost looks like a G1000 with just silly parameters added to the engine tape. i think the top two steam gauges would be RPM and MP. Humidity would be fuel level, etc. Actually everything in that tape is not real.

Here is a real life example of the G1000 PFD...
https://upload.wikimedia.org/wikipedia/commons/thumb/b/b4/Primary_Flight_Display_Garmin_G1000.jpg/1280px-Primary_Flight_Display_Garmin_G1000.jpg

Modern electronics are replacing the vacuum-pump driven AI pictured.
Another example is the Garmin G5, with a 4 hour battery backup.

Boasting a bright, 3.5-inch sunlight-readable liquid crystal display (LCD), the G5 is approved as a primary source for aircraft attitude or turn coordination information and secondary source for altitude, airspeed and vertical speed in a single instrument. Installation is simple and easy as the G5 integrates with the aircraft’s existing pitot/static system to display attitude, airspeed, altitude, vertical speed, slip/skid, turn rate, configurable V-speed references, barometric setting and selected altitude as well as visual alerts upon arriving at the pre-selected altitude. GPS-based track and groundspeed information is also displayed. A dedicated rotary knob also allows for easy adjustments to barometric pressure setting as well as altitude and ground track bugs.

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How long does a runway have to be for an Airbus A380 to takeoff?
I have seen several Airbus A380's takeoff on large runways, but how small can a runway get for an Airbus A380 to takeoff fully without overrunning?

Airbus's A380 AIRCRAFT CHARACTERISTICS AIRPORT AND MAINTENANCE PLANNING data says that for an A380-800 at maximum certified landing weight landing at sea level a 7000 ft (~2150m) runway is required to land (in dry conditions, presumably).
"A380 Briefing For Pilots (Part 2)" shows a 3750m (~12,300ft) takeoff requirement when operating at 600,000kg at sea level, or almost 5000m if operating at 5000ft at 560,000kg (presumably a lower maximum weight for the higher altitude).  If you can significantly cut back on the weight (no PAX, no cargo, minimum fuel), the bottom end of the chart says ~1600m (~5280ft) for "I don't know how I got here, but I need to get away".
(Trying to find better numbers seems to be difficult because the computer does the calculations, and one document I found claiming to be an FCOM (Flight Crew Operations Manual) only describes feeding the data into the computer, and fun things like how the plane can audibly alert "Runway Too Short!" when landing, but I didn't find any takeoff or landing charts in the 8,100 page manual).

Note that MTOW is only 560t, so the value at 600t is not relevant. At 560t at sea level it is a more reasonable ~3000m.

Here is a video of a QANTAS A380 taking off from Los Angeles (LAX) on RWY 24L headed to Sydney (YSSY/SYD).  The aircraft appears to leave the ground at taxiway AA, and using Google Maps' distance measuring tool, the take-off roll distance is 2400 meters / 7875 feet.
For an acceptable safety margin (rejected take-off, etc.), the runway should be at least 500 meters (1640 feet) longer, maybe more.  According to airnav.com and the FAA's downloadable PDF, Runway 24L is 3318 meters / 10885 feet long.
Note that this is a 13-1/2 hour flight, so the aircraft is fairly heavy with fuel and cargo/passengers.

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What is the distance between 2 adjacent waypoints?
I am working on an en route air traffic control system. For the application I plan on allowing the user to plot the position of a few waypoints on a square region which is 100 miles in length and width. The aircraft will then follow a route which passes through the relevant waypoints. Therefore, I would like to know whether there is a general estimate for the distance between 2 adjacent waypoints so that I could have rules for where the waypoints can be plotted and how many of them can be plotted in the whole region.
Thank you very much for the help!

Great circle? Rhumb Line? Vincenty? What world model do you want to use? You would use formulas such as those to find out the distance between two waypoints.

I apologise for not making this clear, but the application is a simulation and for the purpose of the simulation the region will be represented using a cartesian coordinate system. Therefore, the distance between waypoints would just be the euclidean distance between them. So, I just wanted to have a vague estimate for the distance between them in real life for e.g. if it 50-100 miles or 5-10 miles.

Ok, so the difference in position in X/Y cartesian space is just the values subtracted from each other... I'm still not sure what you are getting at? Can you give an example of the coordinates and what you expect the value would be? Why do you want an "estimate" vs the actual value (which is easier to calculate than "estimate")?

Well, the main reason for this question is so that I can put a limit on how many waypoints can be input by the user. The purpose of the application is to show that it will automatically predict future collision between airplanes and make decisions so that the collision can be avoided.  This feature is quite resource hungry. If there are many waypoints then the application would need to do more work to find the final position of the plane after a certain amount of time than it would if the plane was travelling in a straight line throughout its journey.

So to make the simulation realistic and possibly faster I wanted an estimate of the distance between adjacent waypoints in real life. I can use this to set some rules for the user when he/she is plotting waypoints.

Why not go to a site like www.skyvector.com and pick a 100 mile x 100 mile area and see how many waypoints there are.  Airports only? + VORs? + Intersections? +NDBs?  Those would be the waypoints most pilots would use for navigating with.

Okay, I will have a look at the STAR and skyvector.com and use those to decide on a minimum distance between waypoints for my simulation:) Thank you very much. I really appreciate the help!

The distance between waypoints on a procedure or airway can vary tremendously. For waypoins on a high altitude airway, over 100 NM is common. For low altitude Victor airways, less. For an instrument departure or arrival, 10 to 20 miles is common, especially near the terminal area. On an instrument approach, legs are almost always under 10 miles. So the limit for your application depends on what sort of airspace you're modeling.  At 100NM square, that's more nearly the size of a terminal area than an enroute sector, so 10 to 40 miles is probably a reasonable ballpark.
Might be worthwhile to order an actual IFR enroute chart - they're only a few dollars, and then you have a real example of what airspace actually looks like. Or, any of the free flight planning websites can show you essentially the same thing on-line.


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What is the relationship between aspect ratio and divergence speed?
Why do longer wings with higher aspect ratios have lower divergence speeds?

Divergence, as in the speed where shock waves appear?

Because the wing has less torsional stiffness for a given area. This lets lower torsion moments overpower the ability of the wing to resist that torsion.
The torsional stiffness is proportional to the skin thickness and the cross sectional area of the airfoil. For a given wing area and lifting potential, a higher aspect ratio will reduce that cross section, so less stiffness remains.
Divergence occurs when the increase in the aerodynamic torsion moment from the twisting of the wing is bigger than the increase in restoring moment from the wing's torsional stiffness. It does not help that a wider wing has more span to accumulate that twist, so the aerodynamic torsion moment on a high aspect ratio wing is high despite the smaller chord which helps to reduce that torsion moment. However, while chord length is inversely proportional to aspect ratio, the airfoil's cross section is inversely proportional to the square of aspect ratio, so the higher aspect ratio wing has a lower ratio of torsional stiffness to aerodynamic torsion moment.
Theoretically, an aero-isoclinic wing shows no torsion under bending and is supposed to have an infinite divergence speed. However, such a wing must do without ailerons or any other imperfections which might upset the fine balance that is required for its isoclinic nature. The linked report shows that the aero-isoclinic condition is not sufficient to prevent flutter or divergence.


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Is fuel balancing done automatically?
Is the balacing of the fuel in the wings of an aircraft like A320 done automatically or does it have to be done manually by the pilot. If it is done manually does the pilot eyeball the balancing ?

The A320 has an auto mode that will do the following:

Center tank will empty first.
The inner wing tanks will empty until they reach 750 kg in each.
The outer tank transfer valves will open and remain open allowing fuel to flow to the inner tanks.
The center tank pump will stop 5 minutes after low level is reached and remain stopped until the inner wing tank reaches the underfill sensor (approx. 500 kg) at which point it will restart.

The center tank pumps can be switched to manual mode.
The wing tanks can be turned off manually.
The cross feed valve is also operated manually.
source: A320 FCOM

During refueling, it is generally automatic. The fuel handler will just set the amount of fuel to pump in and the pipes will distribute it to the tanks as appropriate.
In flight, there is a schedule which tanks are used first, which is automatic (see TomMcW's answer), but this is not balancing—the left wing tanks only feed engines on the left wing and the right wing tanks only feed engines on the right wing.
If an imbalance appears, meaning the amount of fuel in the left and right wing tanks is significantly different, a warning is generated. There is a cross-feed valve that the pilots can open to let all engines use the fuller tank, but it is not automatic, because unless one engine was shut down in flight (which in twins calls for landing as soon as possible anyway), it usually means the fuel is leaking somewhere and leaving the cross-feed valve open would lead to repeating of the incident of Air Transat flight 236.

On multi-engine jets there is not so much of a need to balance fuel as there is on single-piston aircraft.

The aircraft tends to fly in balance all the time, so there is no differential feed of fuel.
All the engines burn fuel at the same rate, so there is no cause fo a Fuel Imbalance. In fact, a Fuel Imbalance is usually seen as the first symptom of a Fuel leak.


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How do I choose the correct airfoil for my fast cargo UAV project?
I am not able to decide which airfoil to choose for my UAV project as I am confused. I need to lift a maximum of 100kg of extra payload excluding the aircraft's fuel and empty weight. Cruise velocity has a range of 200 to 250kph. Also I would like to know if there is any software or application to decide the airfoil for me?

Aircraft design is iterative. It's never as easy as having a program choose an airfoil for you. For example, the size and shape of your wing is going to factor into your empty weight, which in turn feeds back into what kind of performance your wing needs to have, affecting your choice of airfoil.
Basically, what you need to do is generate a configuration, essentially the layout of your aircraft. Think about how big your fuselage needs to be, where propulsion devices will be located, things like that. Maybe look for inspiration in existing UAVs that have capabilities similar to what your project calls for. Once you have that, you can dig into designing the individual systems.
You can take some guesses at the wing's shape, size, and airfoil. Then you check the performance with computational tools. If it works (it won't the first time), great. If not, try again and improve your design using the information you learned. UIUC maintains a large airfoil database that can give you some ideas of where to start: https://m-selig.ae.illinois.edu/ads/coord_database.html. It sounds like this is an educational project, so you can focus on things like the thickness and camber (degree of curvature) of the airfoil as your iterate.
For example, a thick, highly cambered airfoil would have great takeoff and landing performance, but would stall easily and produce lots of drag in cruise. If I'm finding my aircraft produces high form drag in cruise, I may want to make it thinner to reduce drag. However, be careful not to make the wing so thin that it's too weak to stay attached to the plane (if structural design is important in your project)
Some programs of interest to you might be XFLR5 (http://www.xflr5.com/xflr5.htm) and XFOIL (https://web.mit.edu/drela/Public/web/xfoil/). XFLR5 is popular for hobby aircraft builders and would probably be an appropriate tool for your project, allowing you to analyze an entire 3D aircraft design. XFOIL is for individual 2D airfoil design.
I'll also refer you to Dan Raymer's book, Simplified Aircraft Design for Homebuilders. If you need to build a plane and you don't have an aerospace engineering degree, this is likely the best resource.


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What are these bomb-shaped things attached to some propeller aircraft?

I've seen them on a few prop machines and searched here and via Google but couldn't find anything.

Oh it is indeed, I'm sorry.

That is a weather radar pod.  On multi-engine aircraft, it is typical to install radar in the nose.  But that is not possible in a single engine propeller plane.  So, when radar is installed in one, it is most common to hang it under a wing as shown in the photo.  Here's a link to such a pod

Nice to know, thanks!


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Can turboprops use a recuperator?
A recuperator is a device which extracts heat from the exhaust of a turbine and transfers it to the front air. This heats the air, which means the turbine uses less fuel to heat the combustion chamber to its max operating temperature.
They are used in gas turbines for power generation, because they can supposedly double the fuel economy.
Is this viable in a turboprop? Basically you need 2 heat exchangers per recuperator, so I could understand if they're too heavy. The other thing is potentially blocking airflow too much at the intake, causing too much drag.
Are these problems surmountable in aircraft engines? Is there a way to make them light enough and streamligned enough for aircraft?

The related question doesn't really answer this for turboprops. For jet engines, that related question does explain why it's no good---extracting heat from the exhaust means less jet speed. For turboprops however, it's all about shaft power, just like a electrical generator (before applying power to a prop instead of pure generator).

Has been asked here...  https://aviation.stackexchange.com/questions/37303/why-cant-we-extract-more-from-exhaust-gases-in-gas-turbines/37306#37306

An aircraft engine is a thermodynamic device, that converts heat to mechanical power. A 'virtual' air parcel entering the engine goes through a thermodynamic cycle, which determines the bulk of the power plant efficiency, which include quite a big chuck of irreversibele air friction, with a minor role for bearing losses.
The engineering optimisation process however includes a lot more than just power plant efficiency. At the bottom line, the optimisation is tailored towards generating the most dollars of profit per unit of time, but a more optimistic view is that the engineer will optimise the aircraft fuel efficiency as a whole. In practice, this results in finding a balance between power-to-weight ratio and fuel efficiency, as additional weight increases the power required.
A recuperator is a heat exchanger between the compressor discharge air and the engine exhaust air*. Its goal is to replace part of the energy required from combustion of fuel by energy extracted from the exhaust, and does not modify the thermodynamic cycle in any way. I looked up some (approximated) data from a PT-6 turboprop at 850shp: [source, copied from training manual]

Ambient: 300K
Compressor discharge: 600K
Turbine inlet: 1200K
Exhaust: 800K

A recuperator could in theory cool the exhaust to 600K and heat the compressed air to 800K (ideal infinitely long counterflow heat exchanger) for a delta of 200K. The total desired temperature increase from compressor discharge to turbine inlet is 600K (1200K-600K), so we can reduce the fuel flow required by 1/3rd (only 400K delta still required). That seems excellent!
In practice, heat exchangers are bulky (air is a terrible fluid to transport heat). Furthermore, it's only operating on a 200K difference. The real question then is whether a real-world air-to-air heat exchanger (likely of cross-flow type, like an automotive intercooler, so less efficient than a counter flow heat exchanger) can be made so efficient that it does not introduce more losses than gains. Historic evidence suggest this is unlikely, especially if cost is considered (the material must be able to withstand 800K).
Furthermore, if we're going down the road of heat exchangers, different options are available. For example, an intercooler (cooling between compressor stages, which actually improves the thermodynamic cycle) could work on a higher temperature differential as well as lower absolute temperature, and would only present a single flow obstruction when the air is still nicely compact (power required is volume flow times pressure drop!) inside the engine. However, historic evidence again suggests that this is not cost-efficient, and instead seems to favour increasing the combustion chamber temperature and pressure.
*You may wonder, why not heat the intake air with exhaust air? We would work on a much larger temperature difference! In fact, this would be counterproductive, as the thermodynamic cycle operates on the difference between high and low temperature. Increasing the temperature at the inlet disproportionately increases the power required to compress the air, and does not result in an increase in turbine power (which in the end operates on a pressure differential, and cannot extract work from hot air at ambient pressure). There's a reason we use intercoolers, not inter-heaters in compressors.

Adding heat at the intake is wrong because we get power/thrust from the increase in volume of the high-pressure air exiting the combustion chamber, which can push on more stuff than the small volume of air coming from the compressor. Heating and expanding the high-volume low-pressure air at the intake means there's less expansion we can do in the combustion chamber, since, as the above answer states, there's less temperature difference, thus reducing the power output.

What about connecting the heat exchanger (recuperator) to (inlet and outlet of) the combustion chamber so that the compressor outlet air is heated by the turbine inlet air? This would lower the turbine inlet temperature to compressor exhaust temperature. The temperature delta would be (1200 - 600) = 600K. Would this help for better fuel economy (less fuel for the same thrust)?

@Kozuch That would do effectively nothing because it's just transporting energy round and round in the same step of the thermodynamic cycle. It would lead to a runaway increase in combustion chamber temperature, leading to increased NOx emissions, increased friction and eventual engine failure.


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Foreign EASA 145 Approval Without NAA Approval
I am just trying to wrap my head around a certain matter.
Can a foreign EASA 145 approved organisation release a component if it is based in a country and does not have that country's NAA's approval?
So for example an EASA 145 Approved AMO based in UAE but without GCAA Approval. It just has the Part 145 EASA approval but does not have GCAA CAR 145 approval
Is there any reason why it is allowed or not allowed (Which ever is true).


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Is it permissible to have exit row with ordinary seat spacing?
I was recently flying a flight with an exit row seat on the overwing exit.
I was expecting increased seat spacing for this row but to my surprise the seat spacing seemed exactly the same as rows ahead and behind.
Is this permissible? Are there any rules that mandate minimum seat spacing ( inter row) for exit row seats? I imagine in case of an emergency evacuation it would be very slow for passengers to exit via this row.

Which aircraft type?

define "ordinary" seat spacing. different airlines have different spacings.

In general, the rules are that the aircraft must be evacuated within 90 seconds, with half the exits blocked, in the dark, with smoke in the cabin, with debris obstructing the aisles. If the manufacturer can demonstrate that without having to sacrifice a row of seats, then they are free to do so.

If it wasn’t permissible it wouldn’t have passed certification.

So i am trying to understand the situation. After certification by FAA are airlines allowed to move row spacings? Each airline has different seat spacing by the FAA evacuation trials are only conducted by the aircraft manufacturer for the model right?

@Federico thanks! By ordinary I meant in comparison to other rows on the same aircraft. I was assuming that the emergency row would need to have larger spacing than other rows.


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How do airplanes get stacked in a holding pattern?
This question prompted me to wonder how it works at a major airport. Let's say four airliners all are scheduled to arrive within a few minutes of each other to a major airport, perhaps from the four cardinal directions. Things are already backed up at the airport, let's say landings are (temporarily) restricted due to an emergency landing.
Can you describe how ATC might direct the four flights and what they would do themselves?

ATC has many tools "in their belt" to keep traffic separated. When things back up, it can get crowded. Emergencies and weather regularly cause disruptions and ATC handles them according to the circumstances.
To illustrate some possibilities, I will use Seattle as an example here. You can find the airport information here.
Most aircraft coming into the area will be commercial flights on IFR flight plans, and they will follow a Standard Terminal Arrival (STAR) into Seattle. You can find this seciton towards the bottom of the AirNav page. There are different STARs depending on the direction from which the flight is arriving. Each STAR will include published holding patterns.
Listed here is the direction of arrival, the name of the STAR, and the holding fixes:

East - EPHRATA - FLAAK, EPHRATA, ODESS
East - GLASR - LOSTT, TEMPL, JAKSN
Southeast - CHINS - SUNED, BRUKK
South - OLYMPIA - BTG, OLYMPIA
Southwest - HAWKZ - BTG, HAWKZ
Northwest - JAWBN - JAWBN
Northwest - MARNR - MARNR

As you can see, there are 11 different holding fixes across the seven STARs. If you have one aircraft from each direction, they can each hold at the fix according to their STAR. The busiest STARs have multiple holding fixes that may be used. Aircraft may also be stacked on the same hold at different altitudes. You can read some FAA information about holds here.
Aircraft may also choose to divert. Depending on how much fuel they have and how the weather/emergency is playing out, they may not want to hold indefinitely on the GLASR arrival at LOSTT. Generally ATC will not ask for a diversion unless absolutely necessary. Among others, Portland, Vancouver, and Spokane are all fairly close to where the aircraft may be holding and could be viable alternates.

As @fooot mentions, most, if not all, holding will take place at fixes along the arrival, not within the terminal airspace.  Once airplanes are being controlled by an approach facility they will resolve spacing issues with vectors.  Altitude changes within this airspace can be difficult (see: NY Tracon) and the airspace requirements for holding make it quite inefficient.  Once approach runs out of room to properly sequence with vectors, they'll use their landlines to call the controller at the center feeding the arrivals and tell them "We're closed -- update at 2130Z".
The center will then start the holds.  Aircraft that are closest to being handed off will be assigned the holding at the closest fixes to the destination.  This usually means holding at low altitudes (E.g. holding over Yardley at 8000 ft inbound to EWR).  Aircraft further back will either get a hold close to them, or they'll continue the arrival and hold toward the end.  These decisions are based on airspace availability and how long the anticipated holding will be.  Where center has altitude blocks available, they'll stack aircraft at that fix.  I've been in holding at FL240 where I have seen 10-12 airplanes below me and a few above me, all flying the same racetrack spaced 1000' vertically.  When arrivals resume ATC will peel airplanes off the bottom of the hold, one at at time at intervals dependent on the spacing requested by the approach facility.
If delays become long, aircraft will begin to divert to their alternates, refuel and then wait under a ground stop or ground delay to get back in the air to their final destination.

Do planes that are holding at various places and altitudes get peeled off to land first arrive first land, or does it immediately go to fuel quantities? I wouldn't imagine connections would play apart, right?

First come first serve.
Typically ATC will direct the first plane to the lowest free altitude in the holding pattern and the next 1000 ft on top of that and so on. It may be possible that a plane comes in at a lower altitude right after a plane at a higher one and gets to skip to a lower spot (so it doesn't need to climb up).
When the runway/next segment is available ATC will clear the lowest plane to continue/land and then shift the entire stack down.

Perfect. This is how I imagined it works, but just wanted confirmation.


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Why don't helicopter blades look like other propellers?
While doing an image search, I found many different shaped propellers and asked about them in this question.
In it, Peter Kampf answered my question with his usual thoroughness. However, as I looked at more images of various shaped blades, I saw images of the helicopters of my youth, Bells. The Bell Huey and the Cobra both have very long, very squared off primary blades. They don't have an aerodynamic looking shape and both the leading and trailing edges are straight.
Why would Bell have chosen this shape and not the shape more like a 'normal' plane propeller?

As wingtip evolves for fix-wing aircraft, helicopter's blade tips are less and less square-shaped. Nevertheless, they still are long and thin.

To address your title question: because they are not propellers. They are rotating wings.  See also the common terms "fixed wing" and "rotary wing" as applied to airplanes and helicopters respectively.  Peter covers some of the details with his usual thoroughness.

Helicopter blades need to be long and thin. The diameter of the rotor disc determines the efficiency of the rotor at low speeds and can be compared to the wing span in fixed-wing airplanes. The rotation creates strong centrifugal loads at the blade roots which grow with the square of the tip radius at a given rotation speed, so they cannot be tapered much. Adding chord to the middle of the blade would increase its area and add more friction drag, increasing the torque needed to keep the rotor spinning. If the helicopter is only designed for hover, the blade tips could be tapered, but the added complications of forward flight make a rectangular blade the better choice.
In forward flight, the speeds due to the blade's rotation and the flight speed add up, increasing local airspeed at the advancing blade and reducing it at the receding blade. Since the center of lift is trimmed to be at the rotor hub using the swash plate, the advancing blade has a smaller angle of attack and the receding blade a higher angle of attack. On the receding blade at high flight speed, the root will be in reverse flow since here the rotation speed will not be high enough to compensate for the flight speed. Dynamic pressure is around zero at the middle radius, and only the tip area sees sufficient dynamic pressure to create the lift which is demanded from the blade. Reducing this tip area would reduce the lift which can be produced there, so designers kept the blade chord constant.
Two effects limit the maximum forward speed: The remaining dynamic pressure at the blade tip of the receding blade, and the maximum Mach number at the advancing blade's tip. In order to increase this maximum Mach number, modern rotor tips are swept, but simple sweep will lead to torsion moments which will twist the blade. The pitching moment along the blade needs to be finely balanced so no torsion loads result. To achieve this, helicopter blade airfoils are either symmetric or reflex, and the tip is first swept forward and then backward, so the resulting lift acts exactly at the elastic axis of the blade. See the picture of such a blade tip below. The small trim tab you can see at the trailing edge is used for fine-tuning the pitching moment of the blade.

Eurocopter Blue Edge rotor blade (picture source)
I would not be surprised if an optimized rotor shape has non-rectangular blades, but my impression is that the gains of optimization are small, especially with the wide spread of operation conditions of all rotor blade sections. The rectangular blade is easy to manufacture and good enough for most cases. However, I have not designed a rotor myself and do not know enough about the details to be more specific.

I had seen those blades, that you show and didn't understand the why of it. Thank you for your understandable explanation. I'd accept this, but want to give it a day or three to see if anyone else will chime in.

I've added another answer regarding the shape of heli blades below because it addresses an equally important factor in blade design: autorotation behavior. Both answers must be taken together to answer the OPs question.

Interestingly, this tip shape can be seen on some modern high speed propellers, or 'unshrouded fans', where the tip speed is transonic.

@RobinBennett: Right, and for the same reasons.

There is a second very important consideration for the shape & size of rotor blades: how they behave during an autorotation.
I can think of two factors relating to autorotation in blade design:

the ability of the blades to be driven by airflow up through the blades (the "driven" region of the blade in photo 2), while at the same time generating lift (the "driving" region of the blade):
The amount of inertia stored in the rotating blades which can be expended in the flare by pulling up on the collective to cushion the landing. Small rotors (or group of them, such as in a quadcopter), just don't have enough inertia to nearly eliminate vertical motion in the autorotation flare.

Chapter 11 of the FAA Helicopter Flying Handbook describes heli flight in autorotation.

A helicopter blade must perform on a wider parameter than a propeller designed mostly for thrust.
The straight and or flat blade was found to be good enough for the all the situations that it will encounter and was easy and cheap to produce. Specialized blades can be found but these were matched for the needs such as high speed performance, more quiet blades etc.
One thing that can be said about these special blades is that it uses more exotic materials as normal aluminum honeycombs would not withstand forces produced by such design.


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What is the legal definition of separation as it relates to ATC?
ATC provides "separation" for all aircraft in class bravo airspace. What is the legal definition of separation?
Is it the same or different than the see-and-avoid rule (FAR Part 91.113(b)) where all pilots are supposed to keep their plane from hitting another plane?
If two planes in class bravo airspace were radar identified with a mode C transponder and they hit each other in VMC, who would be responsible for the collision? The pilots or ATC?

For IFR traffic, there are different kinds of separation - separation for wake turbulence, separation for parallel landings, separation from a plane that is shooting an approach, even more so for one that has lost comm's in IMC, and may be breaking of to fly to an alternate airport, etc.  For VFR, when not under ATC control, then it's see & avoid, and IFR traffic is required to see & avoid when in VMC because VFR traffic Not talking to ATC could be around.


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Is the airspace at KUGN class G from the surface to 1200' AGL where class E starts when the tower is closed?
The chart supplement for KUGN here says "AIRSPACE: CLASS D SVC 1200-0200Z other times CLASS E."
Does that mean that when the tower is closed, the class D airspace switches to class E surface area entirely, or is there class G from the surface up to 1200', and class E from 1200 up to 18000'?

SVC means Service, not Surface, that would be SFC. 1200-0200Z is a time range, that means the zone is class D between 12:00 UTC (that's what the Z or Zulu time means) and 0200Z, which would translate to 07:00-21:00 local daylight savings time in Waukegan. Basically, the tower is open during those times and then it closes so it becomes a class E zone, traffic would presumably use unicom if the airport is still open.
The zone is not defined in the sheet, you'd have to look at a chart to get the details on size, shape and how high it goes.

Yep, SVC means "service", but that still begs the question if it is class G or E airspace beginning from the surface of the ground at the airport.

It depends on where you are in the zone @jeff0000, many zones have tiers, for example an airport's zone could be sfc to 3000ft around the airport, then 5nm away it is 1500ft to 3000ft. So if you're talking at the airport then it's class E from the surface, otherwise you need to look at a chart.

This is more of a clarification of the question than an answer…

Per the chart, when the Tower is closed, CTAF is 120.05 (the same freq as Tower was); it is not the same as Unicom on 122.95. This is common at part-time towered airports.

The Chart Supplement will describe only the surface-level airspace. If it says "...other times CLASS E" that means the Class E will begin at the surface. If it says "...other times CLASS G" that means the Class E will not start at the surface, and you will have to check the official airspace definitions (FAA JO 7400.11) to determine if the G goes up to 1200' or only 700'.
You can also check the VFR sectional; you will see that UGN is completely surrounded by a fuzzy-magenta "E starting at 700'" area, so you know that even if the Chart Supplement had listed the surface airspace as Class G the controlled airspace would have started at 700'. But it is possible for an airport to not be surrounded by this fuzzy-magenta marking, and then you really would have no way of knowing where controlled airspace begins without consulting the 7400.11.

Apparently at one point the Chart Supplement did intend to give information regarding the status of the overlying airspace, and actually adopted an entry in the LEGEND to this effect-- which is potentially confusing, because this information never actually appeared in any of the actual airport entries-- see https://aviation.stackexchange.com/q/86583/34686 .Actually though this would pertain to if the entry said "other times Class G".  If it's "other times Class E" it's a moot point because Class E basically has no upper limit.


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Why are some non-towered airports charted with an empty circle?

Guntly Memorial Airport (62C) is shown as an empty circle with four hash marks around the edges. Why is it empty versus some of the other airports around (such as 96C, C89, BUU, etc.) that have a depiction of the runways inside a magenta circle?
Also, do the hash marks around an empty circle mean the usual - that there are fuel services available at that airport?

Your first stop for questions like this should be the FAA Aeronautical Chart User's Guide. In the VFR Sectional & TAC section it says:

Runway patterns will be depicted at airports with at least one hard surfaced runway 1500´ or greater in length.

If you look up the details for 62C in the Chart Supplement or whatever other source you like, you can see it has two grass runways, i.e. no hard surfaced runways.
As for the hash marks, they do mean that fuel is available.

The other airport on this chart that does not have runways is labeled “OBJECTIONABLE”. It may or may not have hard-surface runways but:

The type “OBJECTIONABLE” associated with an airport symbol indicates
  that an objectionable airspace determination has been made for the
  airport per FAA Order JO 7400.2 Section 4, Airport Charting and
  Publication of Airport Data. Objectionable airspace determinations can
  be based upon a number of factors including conflicting traffic
  patterns with another airport, hazardous runway conditions, or natural
  or man-made obstacles in close proximity to the landing area. FAA
  Regional Airports Offices are responsible for airspace determinations.

This designation is covered in this question, although I'm not sure what it has to do with the airport chart symbol?

@Pondlife I was just pointing out that there are some rare cases when the empty circle does not give any indication about the type or length of the runway.


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Why does airnav list frequencies for some STARs on the airport info page?
I was on airnav.com and looked up the info for KIWA. Why does airnav list frequencies for the arrivals (STARs) that serve an airport? Does each frequency correspond to a certain TRACON sector?

They look like the same frequencies that are printed on the STAR charts themselves: 120.7 for DSERT, 124.1 for BLYTHE, etc...


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Does Clearance Delivery operate in the same way for airports in Bravo, Charlie and Delta airspace?
Something tells me that Clearance Delivery might offer different instructions at airports within Bravo, Charlie, and Delta airspace, e.g. if I take off VFR from KLAX vs from a class Delta out in the middle of nowhere.
Can someone give me some guidelines on what might be similar or different? I'm guessing that Clearance Delivery might give me a heading, altitude, and a departure frequency in a class Bravo. In class Delta, I might get an altitude (stay below 2500' AGL) to remain inside of the Delta?

I have seen clearance delivery at B/C only but not at Class D airports. C is just like talking to the ground. I haven't had the courage to land at B yet :)

All will be pretty similar.  In Bravo, and Charlie and Delta if they are close to the Bravo, you will typically contact Clearance for your departure directions (heading to fly, altitude to climb to or hold at after takeoff (or cleared to climb to 1, and expect clearance to higher after so many minutes, or when reaching some intersection), and Approach frequency to expect) before Ground gives you clearance to taxi from parking towards the active runway and for your runup.  If you filed a Flight Plan, expect to have the clearance (perhaps modified) read to you, which you must read back.
After switching to Gnd for Taxi directions, be ready to read those back as well. Have the Airport Diagram out, and don't be afraid to ask for Progressive Directins.  Things can be busy at a Bravo. When your runup is done, you switch to Tower if Gnd hadn't handed you over already, let them know where you are and that you're ready for takeoff.
If the Charlie and Delta are not near the Bravo, things may be a little more relaxed. You may get your Clearance from the Tower, or Gnd, and Gnd/Tower may be the same guy on the same frequency, or he may be on 2 or more frequencies, switching between Gnd and 1 or more Tower frequencies for different air sectors.  You may stay on the Tower frequency longer before being switched to Approach, or for Delta you may just be asked to advise when you are clear of the Delta airspace.
Don't fly to any Bravos until you are comfortable on the radio.  Limit flights to Delta and Charlie to start until you can follow the lingo.

Clearance Delivery in Class B is different for VFR aircraft because it's the only airspace where VFR aircraft require a clearance (rather than merely two-way radio contact) just to be there.  You'll get a specific heading and altitude to use between takeoff and when Tower hands you over to Departure, you can expect one or both to change as soon as you do contact Departure, and you'll keep getting vectors until you're out of their airspace.  You do need to be solid on the radio or they'll kick you out--possibly in the wrong direction if that's most convenient for them; this goes double for arrivals or transits, where they will refuse you entry if you sound like too much of a hassle to deal with.
Class C/D airports are generally more laid back, altitudes are more likely to be "at or above/below" rather than exact (or none at all), you may not be given a specific heading (maybe just "left turn"), and they're usually not in a hurry to get rid of you.  For arrivals and transits, they'll rarely deny you entry unless you sound so incompetent that you shouldn't be flying in the first place.
Class C/D airports under a class B shelf are somewhere in between; in general, Clearance Delivery will send VFR aircraft around the class B rather than through it, but if you're solid on your first contact or two, Departure may decide to clear you through it instead.  If you're not, they'll let you continue on around--and maybe drop you at the next sector boundary instead of handing you off.


text:
Does an aircraft equipment suffix denote how many VOR radios are installed?
I'm studying IFR. Does an aircraft equipment suffix specify if and how many VOR radios are installed?
I'm guessing that the suffix is used by ATC to know if they can give you an RNAV or a an approach that requires DME, so maybe the number of VOR radios is irrelevant? Or maybe ATC assumes that you have a certain number of VOR radios installed?

Are you asking about US Domestic (FAA) or ICAO codes?

I was originally thinking FAA codes.

If you look at the AIM TBL 5−1−3 Aircraft Equipment Suffixes it is split into two parts. The first part has the codes for RVSM which don’t apply to you since I presume you aren’t flying above FL 290. The second part doesn’t mention VORs at all, presumably because historically VOR and DME were almost always used together for IFR navigation. So ATC doesn’t know if you have any VORs based on your equipment suffix. However, if you use /A then they would assume you have VOR capability.
If you have a GPS installed in your plane, then you would be a /G and ATC would know that they can give you an RNAV approach. If you have a GPS installed, then you can use it in lieu of DME. The footnote to Table 1-1-6 gives the conditions when it is applicable.

3 Requires current database or verification that the procedure has not
  been amended since the expiration of the database.

ATC wouldn’t know if your database is current, so it is up to you to accept or reject the clearance.
AOPA has a good article describing when you can use GPS in lieu of ADF and DME.
There is also a StackExchange question that discusses it as well.


text:
Why do low flying aircraft chart areas at night?
Last night in the middle of the night, I kept hearing a low flying aircraft fly overhead (2000ft).  I'm out in the country-side so the noise was very noticeable in the quiet night.  When I checked flightradar24, the aircraft appeared to be moving back and forth in a grid pattern across several small cities in the state and even over a chunk of the country-side.  When I see this during the day, I just assume they are taking pictures for map making.  Why would they be doing this at night?

In short
Tracks, distant of about 300m, covering only cities, Oswego, Fulton, Pulaski and Watertown. It's indeed difficult to be sure, but my guess is the aircraft is conducting thermography, measuring the temperature of the ground to detect fluid network leaks or evaluate buildings insulation.
Local administrations are conducting nowadays thermal surveys to locate poorly insulated buildings. An example of business performing such activity.
Edit: You found the aircraft and the company behind this project, confirming this possibility:

PA-31 registered C-FVZM, owned by Aries Aviation, a company working in remote sensing, and thermography.

Source.

Aries's final customer is MyHeat, a Canadian company providing heat loss maps for housings to customers, with great details.

A paper on this application: Geospatial Technologies to Improve Urban Energy Efficiency

More details
In daytime the Sun light heats the Earth with rays going visible to infrared wavelengths. At any time Earth surface and objects slowly reradiate this heat to the atmosphere, but only in the form of infrared radiation (IR), according to the black body radiation model for temperatures centered on 15°C. IR represent a much larger spectrum than visible light:

Source.
These IR radiations are the ones trapped by GHGs. Human eyes cannot perceived IR, and night appears black to us, but appropriate sensors can see IR, for them night is not dark at all.
To measure energy radiated by Earth surface and objects, it is preferable to work when they don't reflect Sun's light, night is ideal.
Leaks on a city steam network:

Source.
Inventory of heat losses in a city:

Source.
And the same from the company paying for the thermal mapping of your area:

Source.
See The Benefits of Aerial Infrared Thermal Mapping

I think this makes more sense as I've caught 2 other flights (in playback) that focused on Rome, Utica, and Herkimer NY.  The surveys are mainly focused on populated areas, but not always.  I'll try and check adsbexchange overnight and see if I can catch a tail number, it's blocked on flightradar24.  The flights I have seen so far have been every other night.

@rtaft. At least the model, could help check with surveyor fleets. If that was for houses insulation the choice of March-April doesn't seem optimal, so it could be to track leaks from public networks. Do you know is such heat network exist in the towns surveyed?

I didnt realize adsbexchange had playback, Aries Aviation Services Corp. - CFVZM 1969 Piper PA-31

This was conducted by https://myheat.ca/. The utility company in CNY, NationalGrid, is listed as one of the partners/clients.

@rtaft: Great find! MyHeat is doing exactly the same as on the building insulation picture. Do you mind if I use your findings to add these details to my answer?

yes, that is ok

That’s definitely a survey pattern, in addition to photography surveying can use LIDAR to make very accurate maps.

Someone else suggested that to me as well.  He also said it was unusual to do at night since you can't capture the visual light spectrum.  The tail number wasn't available either, idk how common that is though.

@rtaft in the future you can check adsbexchange, it shows a lot more “blocked” tail numbers than commercial sites. But it’s clearly a survey pattern.

Winds are often calmer at night, which would improve accuracy.  LIDAR doesn't need to be aligned to simultaneously recorded visible light images, because GPS is enough.  So you might as well do that at night, when the airplane's otherwise idle.

@CamilleGoudeseune - and likely easier to fly whatever path you want, or at least not be quite as concerned with other traffic.

Sensitive LIDAR systems work better at night because you don't get the extra background photons from the Sun.

@300D7309EF17 thank you for the adsbexchange info, after finding the playback feature, I was able to track down that it was for thermography and all the companies involved in it (as noted in the other answers comments).


text:
What is Transponder Code 2000 actually used for?
On What is the significance of a squawk code? @DeltaLima gave a fairly concise answer. In it he listed several 'standard' codes used for things like emergencies.
Also in the list was Code 2000, for which he included the description:

Used when entering a Secondary Surveillance Area and no code has yet been assigned

So, if I'm in the US, under VFR (code 1200) and I enter a Secondary Surveillance Area, I automatically change to Code 2000? Why wouldn't I have already been given (a specific) designator?

The purpose of squawk code 2000 is to prevent aircraft entering a Secondary Surveillance Radar (SSR) area from transmitting a code that is the same as a discrete code assigned by ATC to an individual aircraft.
If you are flying in the USA under Visual Flight Rules (VFR), you will be assigned (implicitly) code 1200. Upon entering a SSR area you might get assigned another code, but often you will keep code 1200 if you are not near class C airspace.
If you are flying Instrument Flight Rules (IFR) outside a SSR area (e.g. entry into the US from oceanic airspace), you will use code 2000. Upon radar contact you will then get assigned a discrete code.
From FAA Advisory circular 91-70A - (Oceanic and International Operations):

2.  EXPANDED OCEANIC CHECKLIST.
.......
g.
  After Oceanic Entry
(1) Squawk 2000.  Thirty minutes after oceanic en try, crews should
  Squawk 2000, if  applicable. There may be regional differences  such
  as Squawking 2100 in Bermuda’s airspace or  maintaining last assigned
  Squawk in the West Atlantic Route System (WATRS). Crews  transiting
  Reykjavik’s airspace must  maintain last assigned Squawk.

Another use of code 2000 is on the airport: before power up you can select code 2000 to avoid code conflicts that could occur when you would maintain your code from a previous flight. It is standard practice in many airlines to select code 2000 after arrival at the gate.

The SOP written in to every company that I've worked so far is to automatically change to 2000 30 minutes after entering oceanic airspace.  I'm not sure if this is required or not (I don't  have a reference) but it is how the international procedures have been written that I have seen, and I actually haven't had the code specifically assigned to me ever.

@Lnafziger It is usually after 30 minutes indeed, unless the controller assigns you A2000 on handover. See FAA Advisory circular AC 91-70A

That could be useful information to add to your answer.  :)

You'll often hear ATC (Oakland Center or Los Angeles Center) tell aircraft that are heading out over the Pacific something along the lines of "UAL ###, radar services are terminated, squawk 2000, contact ARINC on HF frequencies" when they hand off control to the oceanic ATC.  The 2000 code simply means that the aircraft IS being controlled, but NOT right now by any radar controller.
If they kept whatever squawk they had leaving Oakland or LA center, then there would be the possibility of a conflict as they entered somebody else's radar coverage (Hawaii, Guam, Japan, etc).  So instead of sorting out what the aircraft that are no longer in radar contact are squawking, they simply give them all the standard 2000 code, and when the aircraft is back in radar contact with (whoever), then that controller assigns them a code that works with his center.

The US ATC procedures say that 2000 is to be used for departing IFR aircraft (section 5-2-6):

Code 2000  to an aircraft which will climb to FL 240 or above or to an aircraft which will climb to FL 180 or above where the base of
  Class A airspace and the base of the operating sector are at FL 180,
  and for inter-facility handoff the receiving sector is also stratified
  at FL 180. The en route code must not be assigned until the aircraft
  is established in the high altitude sector.

It looks like the specific information you quoted from the other answer simply doesn't apply in the US. The term "secondary surveillance radar area" isn't in the P/CG and Wikipedia says that the use of 2000 in relation to SSR areas is for ICAO countries only:

The code to be squawked when entering a secondary surveillance radar
  (SSR) area from a non-SSR area used as Uncontrolled IFR flight squawk
  code (ICAO countries)

It does give this information for the US, though:

Non-discrete code assignments in accordance with FAA Order JO 7110.65,
  5-2 *Also for use in oceanic airspace, unless another code is assigned
  by ATC (USA)

The Order it mentions is the ATC instructions. I have no idea about oceanic operations, so I can't comment on that part.
As for VFR, the AIM simply says that all VFR traffic must squawk 1200 unless otherwise instructed by ATC.

Are you distinguishing between "the US" and "ICAO countries"? The USA is one of the original signatories to the Chicago Convention and a full member of ICAO.

@HenningMakholm Yes, that's true, but there's always been a difference between US and ICAO practices, e.g. this recent, well-known example. It's still common to talk about "US vs ICAO" when discussing regulations, even though the US is indeed a member. To be fair, the FAA is more accurate and usually talks about FAA vs ICAO

authors: Síñdhú Ñàìdú


text:
Have landing gear collapsed at the gate?
Reading an answer to this question piqued my curiosity. Has there ever been an instance of an airliner's landing gear collapsing while loading/unloading passengers?
I do not include after emergency landings, but just while "sitting at the gate."

I rather doubt it, it's more likely that it will tip over by badly balanced boarding/loading

There have been quite a few incidents.  Here's one, http://www.nycaviation.com/2013/08/pictures-nose-gear-of-united-boeing-767-300-collapsed-at-gate-at-iah/, Google will find others.

I witnessed a Fokker 50 retracting its gear while parked at the gate. The aircraft made only one flight after that; a ferry flight to the scrapyard. It wasn't loading passengers at the time though.

@Simon Could you please turn your comment into an answer? I cannot accept any of the existing as answers to my question, while your comment is one.

There have been quite a few incidents. Here's one,
http://www.nycaviation.com/2013/08/pictures-nose-gear-of-united-boeing-767-300-collapsed-at-gate-at-iah/
Google will find others.
I can't find links, but do know that there have been incidents caused by different issues such as ground locks not in place, failed struts, WoW logic failures and so on where when hydraulic power was applied, the gear retracted or collapsed.

A World of Warcraft logic failure??? (Newbie: What a WoW logic failure?)

@RoboKaren Sorry, just seen this!  WoW, weight on wheels.  A set of logic which tells the aircraft that it is done flying and firmly on the ground, or alternatively, started flying and no longer firmly on the ground.

A good landing gear design locks it into the extended position under the aircraft's weight. This makes it physically impossible to retract it at the gate, but when the switch is in the wrong position the gear starts to retract during liftoff, when the wings take over the duty of carrying the weight.
This doesn't mean that careless operation or maintenance procedures can't make it retract, however. When the aircraft is poorly balanced, say during loading and unloading, the weight on the nose gear might not suffice to lock it, so a wrong switch position can cause an unintended retraction.

This makes it physically impossible to retract it at the gate - should be This makes it theoretically impossible to retract it at the gate.  There are several instances where it has indeed happened - I linked to one in my comment above.  There are at least 2 others.

@Simon: Please read the 2nd paragraph of my answer.

Landing gear has collapsed at the gate for reasons other than overload on the nose leg.  Your post intimates that it cannot collapse unless poorly balanced which is simply not true.

I did not expect anyone to understand it this way. I spoke of unintended landing gear movement when there is too little load on the gear. This should have been obvious.

Shouldn't the first sentence of the second paragraph read “This doesn't mean that careless operation […] can't make it retract”?

@Relaxed: I Agree - English needs a double negative here. Thanks for the hint!

Not so much in Airliners, but I have seen the result with a GA Aircraft when the pilot was demonstrating to a student (type endorsement) how the squat switch works. Well, squat switch wasn't working, and the gear retracted.. Not pretty..
For reference, a squat switch is attached the the landing gear, and detects when there is pressure on the landing gear (as if there is pressure on the gear, it must be on the ground). If the switch detects pressure, the gear won't retract regardless of the Gear Selector position.

The guy in the hangar next to me had one of his friend's retract the gear on his beautifully restored P-40.  Luckily the mains did not retract, but the tailwheel did and bent quite a bit of metal.  Ouch!


text:
Can a commercial airliner be too heavy to take off?
According to this question's answers, MTOW (or MTOM) is the Maximum Take Off Weight, which is everything up to and including trip fuel. Then there is one step above that, MRW, or Maximum Ramp Weight, which is MTOW plus fuel used to taxi out to take-off.
Can a plane be given too much roll-out fuel, so as the plane is actually too heavy to take off, without having to do perhaps another lap around the airport?
My thoughts are that there are simply no scheduled flights which require a total amount of trip fuel that would make roll-out fuel cause the plane to be too heavy for take off.
EDIT: Although (in a comment) I am asked to possibly change the title of the question to fit more closely (include fuel), after considering it, I've decided to leave it alone. It is unlikely that a plane would be loaded to the point that it is too heavy to take off, before fuel is added. I believe it would be a 100% certainty that it was the addition of fuel that will make (all)(commercial) aircraft go beyond MTOW. There might be some edge cases of contents (payload) being too heavy, but if that were the case the plane would not even be considered for take off (without fuel).

You might edit the title of the question to make it a bit more specific, as the title questions stands it's very general (and the answer is quite clearly "yes", given no constraints).

@JayCarr I think the title's fine. Although the answer to the title question is trivially "yes", the title is only intended to be a guide to what the question is about. If it was always possible to make the title exactly describe the question, you wouldn't need anything else.

@DavidRicherby That's true up to a point, but the title question is pretty far away from what the intent of the question is.  All I'm saying is there are people who may want to ask that exact question, or who may be looking for the answer that exact question, and having this question have that title makes it a little harder for them.

@JayCarr I understand. I've considered it and have added a body edit. I'm leaving the title alone, because I do expect that the answer is yes. What I'm looking for is why and what then. If you can think of a 'better' title, please, do edit it. I'll roll back if I don't like the change.

From a historical standpoint, back in the 1990s, and especially on 747-100/200 cargo flights, it was common to "hide fuel" by specifying more taxi fuel than you knew you would use. Being a few thousand pounds over MTOW was not considered a problem.

They can always dump excess fuel ;)

@Firee dumping fuel on ramp is not a good idea.

It is possible to arrive at the runway with too much weight to takeoff, but this result is generally a mismanagement of fuel.  The most likely scenario this happens is when you are going to be at MGTOW for takeoff but need fuel to get to the runway (putting you over MGTOW at the gate) and are anticipating a long taxi or long lines at a de-ice pad.
If the long taxi doesn't materialize this can result in getting to the runway with too much fuel.  Recognizing this, fixing it is often as simple as taxiing with both engines instead of just one.  If you are really overweight you just ask to get put somewhere you can wait while running both engines and wait for the weight to come down.  There is really no benefit to taxiing around the airport versus just waiting in one spot to burn fuel.

This was exactly the scenario I had in mind, only couldn't come up with a use case (de-icing wait). Also, I didn't really mean a "go around" the airport, but guess I meant perhaps being routed on a return taxiway and looped back.

@CGCampbell ATC usually has somewhere close to the departure end of the runway they can let you wait out of the way.

How frequently might this happen, or is that a whole new question?

@FreeMan not often.  burning fuel is easy.  More often (though still not common) the issue is getting to the runway with close to minimum or not enough fuel (requiring an amendment to the dispatch release).  We are generally quite aware of the fuel situation and make sure to get to the runway with the right numbers.

AF 4950 was 810 kg over the maximum structural weight due to an unexpectedly short taxi prior to take off. Also the fuel was in the aft ballast tank, which should have been empty at take off until supersonic cruise. However the BEA did not think this contributed to the crash.

In addition to de-icing wait, this can happen when, occasionally, fuel delivery personnel simply over fuel an airliner either due to inadequate training, attention, deferred auto systems, or simply mis-reading the order slip.  Either way, some airports cannot pull the fuel back out, others don't want to, and since its a slow process some companies and crews prefer to go out and burn the excess anyway.

Correct on virtually all points; it is possible for an aircraft with high payload or long range expectations to require more fuel be loaded at the gate than it could make it off the runway carrying; however, it wouldn't have to lift off with any fuel it needs to burn just to get to (or even down) the runway. The amount of extra fuel needed would be marginal; a couple hundred pounds at most, just a few minutes' worth at anything more than idle power.
You're also correct that the operator of the plane wouldn't be too eager to test these maximums with any fleet aircraft, and will have their own more conservative guidelines for loading an aircraft. These guidelines are typically only approached by cargo jets; people are relatively low-density given the amount of cabin volume required for each passenger. But, such limits are the reason, for instance, behind airline policies on number of checked bags (and the cost of each one) and the maximum weight of each one; more bags, and/or heavier ones, increase fuel requirements.

I thought bug policies where mostly about turnaround time and airport charges.

@IanRingrose: And airline profits.

The MATOW (max allowable takeoff weight) isn't only limited by its structural weight limit (MTOW) but is absolutely limited by other factors.  For transport category aircraft these other factors can limit max allowable takeoff weight.

Max structural weight
Brake energy for takeoff
Wheel brake cooling limits
Maximum tire speed
Required gradients for first, second and final segment climb to 1500' AGL.
Required runway distance to verify the takeoff weight will produce a runway distance below all runway declared distances.
Obstacles in the takeoff flight path

The pilot in command is required to verify they do not takeoff above any of those limits.  The airlines have dispatchers that will calculate this information.  If a mistake is made, it is possible to be loaded above the max allowable takeoff weight and not realize it.  Most of the list above is accounting for an engine failure below or above the decision speed.
A popular runway analysis program (APG Genesis iPreflight) lists the limiting factors in their app that they use.

Have I understood correctly, that if the runway were e.g. 15 km long, MTOW would be the only limit in practice?

Well no.  The runway length is only one factor to consider.  If it 15 km long, you are still limited by obstacles, tire speed, turn radius, etc.


text:
Why are turbine-powered aircraft "considered ready for takeoff"?
In an answer given to How can I politely remind ATC that I'm waiting for instructions or clearance? by @Mark, he quoted part of AIM regs:

4-3-14. Communications
a. Pilots of departing aircraft should communicate with the control tower on the appropriate ground control/clearance delivery frequency prior to starting engines to receive engine start time, taxi and/or clearance information. Unless otherwise advised by the tower, remain on that frequency during taxiing and runup, then change to local control frequency when ready to request takeoff clearance.
NOTE- Pilots are encouraged to monitor the local tower frequency as soon as practical consistent with other ATC requirements.
REFERENCE- AIM, Automatic Terminal Information Service (ATIS), Paragraph 4-1-13.
b. The tower controller will consider that pilots of turbine-powered aircraft are ready for takeoff when they reach the runway or warm-up block unless advised otherwise.

I have bolded paragraph b because that is what I am asking about here. Why is the tower controller to consider turbine-powered aircraft to be "ready for takeoff when they reach the runway"?
Why are non-turbine engines (piston?) not ready?
What does this paragraph cause the tower to not have to do?

Piston aircraft require a run up, in which the pilot sets the power to a specified RPM, in order to check proper functioning of a variety of systems, and to ensure that the oil is warmed up to a specified temperature. Among the systems to be tested are carborator heat (for normally-aspirated engines), magneto operation, alternate air (for fuel-injected engines), hydraulic systems, and propeller governor (for variable-pitch props). In an aircraft with a hydraulically-activated variable pitch prop, the prop is "exercised" by varying the pitch, which allows the cold motor oil in the prop governor to mix with the warm oil in the engine sump, to bring the entire lubrication system's oil temperature up into the green arc. During the runup, the vacuum system is checked to ensure that enough vacuum pressure is provided to critical instruments. Lastly, a grounding check is made with the magnetos, to make sure that the magneto electrical system is properly grounded.
Performing a run up at the ramp would prove dangerous to people and aircraft if not tied down and due to FOD.
Generally turbine warmup occurs pretty quickly after startup and during taxi. They are usually ready to go at the ramp.

The important point you implied but didn't state explicitly is that turbine aircraft don't need those (nor any other) run-up checks.

Good answer.  Additionally, turbine powered commercial aircraft are generally larger than piston aircraft and can't just turn off into a run-up area to let the more prepared past.  If you aren't ready to go by the time you get to the runway you will delay those behind you.  Not a good way to make friends...

If the engines aren't run up until the aircraft is about to take off, what does a piston aircraft taxi with?

@Vikki If the engine fails on the ground, that's not an emergency. Thus, the plane can taxi without having to do a runup first. But an engine failure in flight is an emergency, thus you want to test the engine (and related systems) with a runup sometime before you take off.

The tower controller can plan that the aircraft will be ready to line up and depart.
This is important in sequencing the traffic at busy airports. If your watching a number of aircraft taxi out to the hold, its likely (dependant on the aerodrome layout) that the piston aircraft will have a number of run up checks to do, which can take a few minutes. Whilst a jet aircraft should be ready to depart.
Certainly in commercial jet ops it's more efficient to be ready to go by the time you reach the hold. Any time spent on the ground decreases the effective utilisation of the aircraft. Controllers are normally aware how keen pilots are to get airborne.
Side note: Jet engines also require a warm up time, this is normally a minimum oil temperature which must be achieved prior to setting takeoff thrust. There is normally no special procedures, warm up being accomplished by the engines being used to taxi the aircraft.

With today's larger turbine engines, there's no "run up" as such except in icing conditions with temperatures below about 3 degrees Celsius. Under more normal conditions, both General Electric and Pratt & Whitney engines powering today's biggest jets need about 3 minutes and 5 minutes respectively, of no more than taxi thrust levels and oil temperature off (above) the bottom mark to permit take off. On some, the procedures even mention that the take off need not be delayed on account of these running times as long as the oil pressure is registering.
There's very little else or nothing by way of engine checks before departure. Secondary engine parameters such as oil temp and pressure are normally not even displayed except during start and automatically in case the numbers start changing to a 'not normal' value.
Turbines tend to be mechanically far less complex than reciprocating engines and less prone to failure. Keeping in mind these almost invisible engine operating procedures, ATC expects them to come up to the runway in sequence and accept take of clearance without delay.

Welcome to [aviation.se], skipper44! Thank you for your useful answer. I have edited it slightly, and please feel free to roll back my edits if you disagree with them. You might find it helpful to review the site [tour] and the [help]. Thanks!


text:
How does ATC ask a pilot to resume flying a published SID / STAR?
Let's say that you are cleared for and are flying the ZZOOO2 departure out of KSAN and you get vectored off of the departure before you complete it.
At some point, ATC wants to put you back onto the departure.
How would ATC ask you to get back on the ZZOOO2 departure, adhering to all altitude and speed restrictions along the way? Is there standard phraseology that exists?

More than likely they give you vectors to fly to get you back on course, or have you proceed directly to the next waypoint. Really depends on where you got turned away and your intended direction of flight afterwards.


text:
Did Southwest airlines retire flight 1380 following the accident, and with what number?
Just wondering if Southwest Airlines flight 1380 was retired and replaced with a different flight number following the uncontained engine failure and depressurization accident in April 2018?

According to FlightAware, Southwest has not operated a flight with flight number 1380 since the accident flight on April 17. The flights with that number for several weeks after the accident were all been marked as cancelled in the FlightAware database; other flight numbers were used:

Southwest uses the same flight number for many flight segments in a day, but their flight schedule shows that flights from LGA to DAL leaving at around 10:30am are using the flight numbers 5227 (Sundays, 10:35) and 237 (weekdays, 10:45).

It appears so, as there is no longer a flight 1380 from LGA to DAL.